Heterojunction Bipolar Transistor (HBT) epi wafer manufacturing at thousands of wafers per week requires careful monitoring and control to ensure excellent quality and high yields. All product epi wafers undergo non-destructive testing which gives data on epi wafer sheet resistance, particle density, and surface roughness. These non-destructive measurements give only a limited view of wafer quality, therefore periodic destructive testing of selected epi wafers (HBT device fabrication and measurement) is required. It is highly desirable to directly measure key epi layer parameters on each product wafer in order to continue pushing toward the goals of perfect quality and yields. Therefore we have been working with the Laytec EpiTT in situ optical reflectance monitoring system. These data allow for immediate detection of shifts in the epi layers. Therefore problems can be detected and corrective actions taken before failed wafers are grown.